(1) Field of the Invention
The invention relates to a current sense circuit for a power output, and more particularly, to a current sense circuit using a novel sensing current matching circuit and using a current controlled oscillator to generate a frequency proportional to the output current.
(2) Description of the Prior Art
Current sensing circuits find many applications. In power switching circuits, it is desirable to know the load current so that over-current protection can be performed. In addition, it is useful to know the load current in some applications, such as in fuel gauge circuits, where the current level must be controlled. It is further desirable to accurately determine the load current without the addition of sensing resistance in the load current path.
Several prior art inventions describe current sensing circuits. U.S. Pat. No. 5,821,740 to Hodgins et al describes a DC-to-DC converter using a sensing transistor in parallel with an output driver. A sense resistor is driven by the sensing transistor and is used to detect over-current conditions. U.S. Pat. No. 5,680,034 to Redl and U.S. Pat. No. 6,201,417 to Blum et al teach pulse width modulation (PWM) controllers using a current sense resistor and an S-R latch for current limiting.
A principal object of the present invention is to provide an effective and very manufacturable current sense circuit for a power switch output.
A further object of the present invention is to provide a current sense circuit where a sense current is generated using a sense transistor.
A yet further object of the present invention is to improve the accuracy of the sense current by equalizing the drain to source voltage for the power transistor and the sense transistor.
Another further object of the present invention is to provide a digital frequency signal in proportion to the power switch output current using a current controlled oscillator.
Another still further object of the present invention is to provide a current sense circuit with a high dynamic range and excellent linearity.
Another still further object of the present invention is to provide a sense circuit with an adjustable sense factor.
In accordance with the objects of this invention, a current sense circuit is achieved. The circuit comprises, first, an output transistor having gate, source, and drain. The drain is coupled to a load, the source is coupled to a power rail, and the gate is coupled to a control voltage such that the output transistor conducts an output current. Second, a sense transistor has gate, source, and drain. The source is coupled to the power rail, and the gate is coupled to the control voltage. A sensing factor comprises the output transistor size divided by the sense transistor size. Third, a means of equalizing the sense transistor drain-to-source voltage and the output transistor drain-to-source voltage is used such that the sense transistor drain current comprises the output current divided by the sensing factor. Finally, a current controlled oscillator is included. The current controlled oscillator has input and output. The input comprises the sense transistor drain current. The output comprises a cyclical signal having a frequency proportional to the output current.